This invention relates to abrasion resistant coatings and specifically to easily scratched substrates with an abrasion resistant coating deposited thereon.
For many purposes artificial polymers have been developed to take the roles traditionally filled by very durable materials such as glass and metal. In many ways the replacements are superior but frequently they have the drawback that they suffer far too readily from abrasion damage. For this reason much work has been done on making the polymers harder and tougher.
There is however, an alternative approach that in some situations is preferred. This comprises the deposition of an abrasion resistant coating on the surface. The present invention falls into this latter category.
One such application which is becoming of increasing significance is in the field of optical laminates, that is transparent sheets comprising at least two layers of different materials and for the sake of clarity and brevity, this application is described in terms of that end use. It is obvious however that the coating compositions described herein can be used on other than laminates and on shaped articles such as bottles. All such uses are to be understood as being within the reasonable purview of this invention.
In certain applications notably in car windshields, optical laminates comprising a thermoplastic sheet material between two sheets of glass are installed as standard equipment in many countries to comply with legislated safety standards. Such laminates have excellent strength and resistance to impact but do tend to be thicker and heavier than is desirable for this application especially when car weights are being cut to improve gas mileage. Moreover, the impact of an object on the surface of the inboard, that is the passenger side, surface of the windshield which is sufficient to splinter the glass layer is usually followed by a sliding downward movement along the fragments of splintered glass. Where the object making the impact is a human head, the resultant lacerations can be extremely unpleasant. It has, therefore, been proposed that the inboard glass layer be replaced by a lighter, thinner thermoplastic material that will not splinter to produce sharp cutting edges on impact.
Suitable replacement materials that have been proposed include polycarbonate, polyethylene terephthalate, polyurethane, cellulose aceto-butyrate, acrylates and styrene and acrylonitrile copolymers. Unfortunately, all of the suitable polymers so far described are much inferior to glass from the point of view of abrasion resistance so that in use the inboard surface is easily damaged by casual contact and even, in some cases, in normal cleaning operations.
One solution to this problem is to provide the inboard layer of thermoplastic material with an abrasion resistant coating. This is a subject that, in its broadest form, i.e., the provision of coatings for easily scratched thermoplastic materials has exercised inventive minds since the mid 1940s.